1. Field of the Invention
The invention relates to a projection display, more particularly to one that includes three polarization beam splitter prisms.
2. Description of the Related Art
In a conventional projection display, beam splitter prisms are employed to separate a light beam into red, blue and green primary color components to facilitate independent control of the characteristics of the latter and to achieve the effect of optimum contrast. FIG. 1 illustrates a conventional projection display 1 that incorporates four polarization beam splitter prisms. In the conventional projection display 1, a P-polarization light beam 2 is processed before providing the same to a projection lens 20. The projection lens 20 has an optical axis that is parallel to an input axis of the light beam 2. The light beam 2 includes a first (green) color component 21, a second (blue) color component 22, and a third (red) color component 23.
In order to achieve independent control of the color components 21, 22, 23, the components of the conventional projection display 1 are generally arranged into first and second rows. The first row is aligned with the input axis of the light beam 2. The second row is aligned with the optical axis of the projection lens 20. The components in the first row, in an order from left to right and with the light beam 2 coming from the left, include a first light polarization selector 10, a first polarization beam splitter prism 11, a second light polarization selector 12, a second polarization beam splitter prism 13, and a first reflective light valve 14. The components in the second row, in an order from left to right and with the projection lens 20 disposed at the right, include a second reflective light valve 15, a third polarization beam splitter prism 16, a fourth polarization beam splitter prism 17, and a third light polarization selector 18. A fourth light polarization selector 19 is disposed between the second and fourth polarization beam splitter prisms 13, 17. A third reflective light valve 29 is disposed on one side of the second polarization beam splitter prism 13 opposite to the fourth light polarization selector 19.
The operation of the conventional projection display 1 when the first, second and third reflective light valves 14, 15, 29 are in an active (ON) state is as follows: When the P-polarization light beam 2 passes through the first light polarization selector 10, the polarization state of the first color component 21 will be changed from P-polarization to S-polarization. Thereafter, the S-polarization first color component 21 will be reflected by the first polarization beam splitter prism 11 toward the third polarization beam splitter prism 16, and will be further reflected by the third polarization beam splitter prism 16 toward the second reflective lightvalve 15. Because the second reflective light valve 15 is in the active (ON) state, the polarization state of the first color component 21 will be changed from S-polarization to P-polarization, and the P-polarization first color component 21 will be reflected back to the third polarization beam splitter prism 16. The P-polarization first color component 21 then passes through the third polarization beam splitter prism 16, the fourth polarization beam splitter prism 17, and the third light polarization selector 18 such that the polarization state of the first color component 21 is changed to S-polarization prior to reaching the projection lens 20.
As the P-polarization first color component 21 passes through the third and fourth polarization beam splitter prisms 16, 17, about 10% thereof will form polarized light leakage components 211, 212. However, the polarized light leakage components 211, 212 have reflected paths that are transverse to the optical axis of the projection lens 20 and therefore do not reach the projection lens 20. As such, when the second reflective light valve 15 is in the active (ON) state, the polarized light leakage components 211, 212 that are attributed to the first color component 21 will not be projected by the projection lens 20 so as not to affect the output image quality.
After the P-polarization second and third color components 22, 23 pass through the first light polarization selector 10, they will be allowed to pass directly through the first polarization beam splitter prism 11 so as to reach the second light polarization selector 12. As the P-polarization second color component 22 passes through the second light polarization selector 12, the polarization state thereof will change from P-polarization to S-polarization. Thereafter, the S-polarization second color component 22 will be reflected by the second polarization beam splitter prism 13 toward the third reflective light valve 29. Because the third reflective light valve 29 is in the active (ON) state, the polarization state of the second color component 22 will be changed from S-polarization to P-polarization, and the P-polarization second color component 22 will be reflected back to the second polarization beam splitter prism 13. The P-polarization second color component 22 then passes through the second polarization beam splitter prism 13 so as to reach the fourth light polarization selector 19. As the P-polarization second color component 22 passes through the fourth light polarization selector 19, the polarization state thereof will change from P-polarization to S-polarization prior to reaching the fourth polarization beam splitter prism 17. The fourth polarization beam splitter prism 17 reflects the S-polarization second color component 22 to enable the latter to pass through the third light polarization selector 18 and reach the projection lens 20.
As the P-polarization second color component 22 passes through the second polarization beam splitter prism 13, a polarized light leakage component 221 will be formed. However, since the polarized light leakage component 221 is reflected by the second polarization beam splitter prism 13 so as not to reach the projection lens 20, the output image quality will not be affected thereby.
The P-polarization third color component 23 from the first light polarization selector 10 will be allowed to pass through the first polarization beam splitter prism 11, the second light polarization selector 12 and the second polarization beam splitter prism 13 so as to reach the first reflective light valve 14. Because the first reflective light valve 14 is in the active (ON) state, the polarization state of the third color component 23 will be changed from P-polarization to S-polarization, and the S-polarization third color component 23 will be reflected back to the second polarization beam splitter prism 13. The S-polarization third color component 23 is then reflected by the second polarization beam splitter prism 13 so as to pass through the fourth light polarization selector 19 and reach the fourth polarization beam splitter prism 17. The fourth polarization beam splitter prism 17 reflects the S-polarization third color component 23 to pass through the third light polarization selector 18 prior to reaching the projection lens 20.
As the P-polarization third color component 23 passes through the first and second polarization beam splitter prisms 11, 13, polarized light leakage components 231 will be formed. However, since the polarized light leakage components 231 are reflected so as not to reach the projection lens 20, the output image quality of the conventional projection display 1 will not be affected thereby.
Referring to FIG. 2, the paths of the color components 21, 22, 23 of the light beam 2 when the first, second and third reflective light valves 14, 15, 29 of the conventional projection display 1 are in an inactive (OFF) state are as follows:
1. The P-polarization first color component 21 will pass through the first light polarization selector 10 such that the polarization state thereof will be changed to S-polarization. The S-polarization first color component 21 will then be reflected by the first polarization beam splitter prism 11 toward the third polarization beam splitter prism 16, and will be further reflected by the third polarization beam splitter prism 16 toward the second reflective light valve 15. The second reflective light valve 15 will reflect the S-polarization first color component 21 back to the third polarization beam splitter prism 16, and the third polarization beam splitter prism 16 will reflect the S-polarization first color component 21 back to the first polarization beam splitter prism 11. The S-polarization first color component 21 will then be reflected by the first polarization beam splitter prism 11 to pass through the first light polarization selector 10.
2. The P-polarization second color component 22 will pass in sequence through the first light polarization selector 10, the first polarization beam splitter prism 11, and the second light polarization selector 12. When the P-polarization second color component 22 passes through the second light polarization selector 12, the polarization state of the second color component 22 will change from P-polarization to S-polarization. Thereafter, the S-polarization second color component 22 will be reflected by the second polarization beam splitter prism 13 toward the third reflective light valve 29. The S-polarization second color component 22 will be reflected by the third reflective light valve 29 back to the second polarization beam splitter prism 13, which in turn reflects the S-polarization second color component 22 to enable the latter to pass through the second light polarization selector 12 and reach the first polarization beam splitter prism 11. Finally, the first polarization beam splitter prism 11 reflects the S-polarization color component 22 away from the projection lens 20.
3. The P-polarization third color component 23 will pass in sequence through the first light polarization selector 10, the first polarization beam splitter prism 11, the second light polarization selector 12, and the second polarization beam splitter prism 13 so as to reach the first reflective light valve 14. The first reflective light valve 14 then reflects the P-polarization third color component 23 so as to pass in sequence through the second polarization beam splitter prism 13, the second light polarization selector 12, the first polarization beam splitter prism 11 and the first light polarization selector 10.
It has thus been shown that the four polarization beam splitter prisms 11, 13, 16, 17 and the four light polarization selectors 10, 12,18, 19 of the conventional projection display 1 cooperate to prevent light leakage components from reaching the projection lens 20 regardless of the operating states of the reflective light valves 14, 15, 29, thereby ensuring optimum image contrast quality. However, the use of four polarization beam splitter prisms 11, 13, 16, 17 results in higher costs, and in a bulky and heavy construction.
Therefore, the object of the present invention is to provide a projection display that utilizes three polarization beam splitter prisms to result in lower costs and in a compact and lighter construction without adversely affecting the image contrast quality.
According to the present invention, a projection display is used to process a light beam that is to be provided to a projection lens and that includes first, second and third color components. The projection display comprises a dichroic beam splitter prism, a first polarization beam splitter prism, a second polarization beam splitter prism, a third polarization beam splitter prism, a first light polarization selector, a second light polarization selector, a third light polarization selector, a first reflective light valve, a second reflective light valve, and a third reflective light valve.
The dichroic beam splitter is adapted to separate the first color component from the second and third color components.
The first polarization beam splitter prism is disposed adjacent to the dichroic beam splitter so as to receive the first color component therefrom.
The second polarization beam splitter prism is disposed adjacent to the dichroic beam splitter so as to receive the second and third color components therefrom.
The third polarization beam splitter prism is disposed adjacent to the first and second polarization beam splitter prisms.
The first reflective light valve is disposed adjacent to the first polarization beam splitter prism so as to receive the first color component therefrom. The first reflective light valve processes the first color component from the first polarization beam splitter prism by changing the polarization state of the first color component when in an active state, and reflects the first color component back to the first polarization beam splitter prism. The first polarization beam splitter prism provides the first color component processed by the first reflective light valve to the third polarization beam splitter prism.
The first light polarization selector is disposed between the dichroic beam splitter and the second polarization beam splitter prism for changing the polarization state of one of the second and third color components so as to enable the second polarization beam splitter prism to separate the second color component from the third color component.
The second reflective light valve is disposed adjacent to the second polarization beam splitter prism so as to receive the second color component therefrom. The second reflective light valve processes the second color component from the second polarization beam splitter prism by changing the polarization state of the second color component when in an active state, and reflects the second color component back to the second polarization beam splitter prism. The second polarization beam splitter prism provides the second color component processed by the second reflective light valve to the third polarization beam splitter prism.
The third reflective light valve is disposed adjacent to the second polarization beam splitter prism so as to receive the third color component therefrom. The third reflective light valve processes the third color component from the second polarization beam splitter prism by changing the polarization state of the third color component when in an active state, and reflects the third color component back to the second polarization beam splitter prism. The second polarization beam splitter prism provides the third color component processed by the third reflective light valve to the third polarization beam splitter prism.
The second light polarization selector is disposed between the second and third polarization beam splitter prisms for changing the polarization state of one of the second and third color components that passes from the second polarization beam splitter prism to the third polarization beam splitter prism.
The third polarization beam splitter prism is further disposed so as to direct the first, second and third color components received from the first and second polarization beam splitter prisms to the projection lens.
The third light polarization selector is to be disposed between the third polarization beam splitter prism and the projection lens. The third light polarization selector ensures that the first, second and third color components from the third polarization beam splitter prism have the same polarization state prior to reaching the projection lens.